AMP deaminase originating streptomyces and utilization thereof

ABSTRACT

It is intended to provide a thermostable AMP deaminase originating in a microorganism. Namely, an AMP deaminase having the following characteristics. (1) Catalyzing the reaction: 5′-adenylic acid+H 2 O→5′-inosinic acid+NH 3 ; (2) being stable at a temperature of 65° C. or below (in an acetate buffer (pH 5.6)); (3) having a molecular weight of 48,000±2,000 in gel filtration; and (4) having the optimum pH value at around 5.6 (in McIlvaine buffer).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase application filed under 35 U.S.C. §371 from PCT Application No. PCT/JP2005/007892, filed Apr. 26, 2005, which claims priority to Japanese Patent Application No. JP 2004-134464, filed Apr. 28, 2004, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to AMP deaminase. More particularly, the present invention relates to AMP deaminase derived from a microorganism.

BACKGROUND ART

AMP deaminase is also called adenyl deaminase, AMP aminohydrolase, and the like, and catalyzes a reaction of allowing adenylic acid to hydrolytically deaminate to generate inosinic acid and ammonia. It has been reported that AMP deaminase has been widely found in living animal tissues and separated from various tissues from various species to date (Fujishima T. and Yoshino H., Amino Acid-Nucleic Acid, Vol. 16, pp 45-55 (1967): non-patent document 1, Magdale'na Rosinova' et al., Collection Czechoslov. Chem. Commun. Vol. 43, pp 2324-2329 (1978): non-patent document 2, Japanese Patent Unexamined Publication No. S55-120788: patent document 1). Meanwhile, mainly from the viewpoint of industrial utilization, search for AMP deaminases derived from microorganism has been extensively carried out. Particularly, many studies have been done on AMP deaminase derived from filamentous bacterium. Some AMP deaminases, for example, AMP deaminase derived from Aspergillus melleus have been attempted to be industrially used aiming at increasing taste in production of yeast extract.

-   Patent document 1: Japanese Patent Unexamined Publication No.     S55-120788 -   Non-patent document 1: Fujishima T. and Yoshino H., Amino     Acid-Nucleic Acid, Vol. 16, pp 45-55 (1967) -   Non-patent document 2: Magdale'na Rosinova' et al., Collection     Czechoslov. Chem. Commun. Vol. 43, pp 2324-2329 (1978)

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

At present, in the production of yeast extract, in order to increase taste, nuclease and AMP deaminase are generally used. In general, the optimum temperature of nuclease is about 65° C. On the other hand, the optimum temperature of a currently used AMP deaminase from Aspergillus melleus is about 50° C. Therefore, in production, it is impossible to allow two enzymes to act simultaneously at high temperature. Treatment with nuclease and treatment with AMP deaminase had to be carried out separately. If AMP deaminase has an excellent thermal resistance property, it can be allowed to act together with nuclease. Thus, production process can be shortened. Furthermore, since treatment with these two enzymes can be carried out at high temperatures, it is not necessary to once reduce the treatment temperature to about 50° C. that is a reaction temperature of AMP deaminase. Thus, it is possible to prevent contamination effectively. Thus, in the industrial utilization, a thermal resistant AMP deaminase has many advantageous and it has been demanded to be found.

Means to Solve the Problems

Under the above-mentioned circumferences, the present inventors have carried out a screening for searching microorganisms for a novel AMP deaminase. As a result, the present inventors have found that Streptomyces of the genus Streptomyces produces AMP deaminase having a high thermostability. Furthermore, they have obtained the findings that AMP deaminase produced by Streptomyces murinus has an excellent thermostability.

After the present inventors obtained the above-mentioned findings, they attempted to identify the enzyme (AMP deaminase derived from Streptomyces murinus). As a result, as shown below, they succeeded in identifying the enzyme and clarified the amino acid sequence and nucleotide sequence thereof, which has enabled the production of the enzyme as a recombinant protein. Furthermore, the use of a technique such as gene recombination has enabled the productivity of the enzyme to be enhanced and the enzyme itself to be improved.

The present invention was made based on the above-mentioned findings, and provides the following configurations.

[1] AMP deaminase comprising the following characteristics:

(1) catalyzing a reaction: 5′-adenylic acid+H₂O→5′-inosinic acid+NH₃;

(2) being stable at a temperature of 65° C. or less (in an acetate buffer solution (pH 5.6));

(3) having a molecular weight of 48,000±2,000 in gel filtration and 60,000±3,000 in SDS-PAGE; and

(4) having an optimum pH of around 5.6 (in a McIlvaine buffer solution).

[2] AMP deaminase derived from Streptomyces, comprising the following characteristics:

(1) catalyzing a reaction: 5′-adenylic acid+H₂O→5′-inosinic acid+NH₃; and

(2) being stable at a temperature of 65° C. or less (in an acetate buffer solution (pH 5.6)).

[3] The AMP deaminase described in [2], wherein the Streptomyces belongs to the genus Streptomyces.

[4] The AMP deaminase described in [2], wherein the Streptomyces is selected from the group consisting of Streptomyces murinus, Streptomyces celluloflavus, and Streptomyces griseus.

[5] A method of producing yeast extract, the method comprising a step of allowing the AMP deaminase described in any one of [1] to [4] to act.

[6] A method of producing a taste substance by allowing the AMP deaminase described in any one of [1] to [4] to act on 5′-nucleotide so as to deaminate the 5′-nucleotide.

[7] A method of producing AMP deaminase, the method comprising:

culturing Streptomyces of the genus Streptomyces in a nutrient medium so as to produce the AMP deaminase described in [1]; and

collecting the produced AMP deaminase.

[8] The method described in [7], wherein the Streptomyces is selected from the group consisting of Streptomyces murinus, Streptomyces celluloflavus, and Streptomyces griseus.

[9] An isolated AMP deaminase consisting of the following (a) or (b):

(a) a protein having an amino acid sequence set forth in SEQ ID NO: 1;

(b) a protein having an amino acid sequence obtained by modifying a part of the amino acid sequence set forth in SEQ ID NO: 1, and functioning as AMP deaminase.

[10] An isolated nucleic acid molecule encoding the AMP deaminase described in [9].

[11] The isolated nucleic acid molecule described in [10], having any one of the following nucleotide sequences (a) to (c):

(a) a nucleotide sequences of any one of SEQ ID NOs: 3 to 5;

(b) a nucleotide sequence obtained by modifying a part of the nucleotide sequence described in (a), and encoding a protein functioning as AMP deaminase; and

(c) a nucleotide sequence hybridizing a nucleotide sequence complementary to the nucleotide sequence described in (a) or (b) under stringent conditions, and encoding a protein functioning as AMP deaminase.

[12] A vector carrying the nucleic acid molecule described in any one of [9] to [11].

[13] A transformant in which the nucleic acid molecule described in any one of [9] to [11] is introduced.

[14] A method of producing AMP deaminase, the method comprising the following steps (1) and (2):

(1) culturing the transformant described in [13] in a condition capable of producing a protein encoded by the nucleic acid molecule; and

(2) collecting the produced protein.

EFFECT OF THE INVENTION

AMP deaminase of the present invention has excellent thermostability and can act at relatively high temperatures. Therefore, the enzyme reaction can be carried out in the conditions free from contamination. Furthermore, AMP deaminase of the present invention can be acted simultaneously with other enzymes acting at high temperature, for example, nuclease used in the production of yeast extract, and therefore simplification and shortening of the production process can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the comparison of thermostabilities of AMP deaminase produced by Aspergillus melleus and Streptomyces murinus. In this graph, the abscissa represents a reaction temperature and the ordinate represents the residual deaminase activity (%).

FIG. 2 is a table showing the comparison of phosphatase activity/deaminase activity (P/D) of AMP deaminases produced by Aspergillus melleus and Streptomyces murinus.

FIG. 3 is a graph showing the comparison of IMP conversion rate of AMP deaminases produced by Aspergillus melleus and Streptomyces murinus.

FIGS. 4 (a) and (b) are graphs showing the comparison of IMP conversion rate in a case where AMP deaminase derived from Streptomyces murinus is allowed to act in the production process of yeast extract. FIG. 4 (a) shows the measurement results in the case where the nuclease treatment and the deaminase treatment are carried out separately as in the present production process. FIG. 4 (b) shows the measurement results in the case where the nuclease treatment and the deaminase treatment are carried out simultaneously.

FIGS. 5 (a) and (b) are graphs showing chromatography results by the purification process of AMP deaminase derived from Streptomyces murinus. FIG. 5 (a) shows a chromatography result by HiPrep™ 16/10 ButylFF, and FIG. 5 (b) shows a chromatography result by Superose 12.

FIG. 6 (a) is a table summarizing total enzyme activity, total protein amount, specific activity, and yield in the purification process of AMP deaminase derived from Streptomyces murinus. FIG. 6 (b) shows results of analysis of purified enzyme by SDS-PAGE (CBB staining). Lane I shows bands of protein molecular weight marker. Bands of phosphorylase b (M.W. 97,400), bovine serum albumin (M.W. 66,267), aldolase (M.W. 42,400), carbonic anhydrase (M.W. 30,000), trypsin inhibitor (M.W. 20,100), lysozyme (M.W. 14,400) are shown sequentially from the side of high molecular weight.

FIG. 7 (a) is a graph showing a relation between a reaction temperature and activity of AMP deaminase derived from Streptomyces murinus. FIG. 7 (a) shows the relative activity (%) when the activity value in the reaction at 65° C. is defined as 100%. FIG. 7 (b) is a graph showing the thermostability of AMP deaminase derived from Streptomyces murinus.

FIG. 8 (a) is a graph showing a relation between pH and activity of AMP deaminase derived from Streptomyces murinus. FIG. 8 (a) shows the relative activity when the activity value in the reaction in pH 5.6 is defined as 100%. FIG. 8 (b) is a graph showing the pH stability of AMP deaminase derived from Streptomyces murinus.

FIG. 9 is a graph summarizing substrate specificities of AMP deaminase derived from Streptomyces murinus. FIG. 9 shows relative activities when the enzyme activity with respect to AMP is defined as 100%.

FIG. 10 is a graph showing results of chromatofocusing analysis of AMP deaminase derived from Streptomyces murinus.

FIG. 11 is a graph summarizing characteristics of AMP deaminase derived from Streptomyces murinus. For comparison, optimum pH, etc. of nuclease derived from Penicillium citrinum and AMP deaminase derived from Aspergillus meleus are shown.

FIG. 12 is a table showing the comparison of thermostability and substrate specificity of AMP deaminase produced by Streptomyces griseus subsp. griseus, Streptomyces griseus, and Streptomyces celluloflavus.

FIG. 13 shows analysis results of the N-terminus amino acid sequence and the internal amino acid sequence of AMP deaminase derived from Streptomyces murinus as follows: N-terminus-A P P E E Q A T D A E E R T D-C-terminus (SEQ ID NO:27); TD DAGYL (SEQ ID NO: 28); N-terminus-E A D D G D A E F R-C-terminus (SEQ ID NO: 29): and F G B V T A R H R G-C-terminus (SEQ ID NO: 30).

FIG. 14 shows a restriction map of AMP deaminase derived from Streptomyces murinus.

FIG. 15 shows a construction procedure of shuttle vector pSV1.

FIG. 16 shows a construction procedure of AMP deaminase expression vector pSVSAD.

FIG. 17 is a table showing the measurement result of activity of AMP deaminase produced by transformant SAD-1 into which an AMP deaminase gene is introduced. ND denotes “not detected”.

FIG. 18 is a graph showing thermostability of AMP deaminase produced by transformant (SAD-1). The abscissa represents a reaction temperature and the ordinate represents residual deaminase activity (%).

FIG. 19 is a table summarizing substrate specificities of AMP deaminase derived from transformant (SAD-1). FIG. 19 shows relative activities when the enzyme activity with respect to AMP is defined as 100%.

FIG. 20 shows results of analysis of AMP deaminase derived from transformant (SAD-1) by SDS-PAGE (CBB staining). Lane II is a control sample lane (culture supernatant of host), and lane III is a sample lane (culture supernatant of transformant). Lane I shows bands of protein molecular weight marker. Phosphorylase b (M.W. 97,000), bovine serum albumin (M.W. 66,000), ovalbumin (M.W. 45,000), carbonic anhydrase (M.W. 30,000), trypsin inhibitor (M.W. 20,100) bands are shown from the side of the high molecular weight.

FIG. 21 shows SEQ ID NO: 32 an amino acid sequence or AMP deaminase derived from Streptomyces which has been successfully identified and SEQ ID NO: 31 a sequence (including a promoter region and a terminator region) coding therefor.

FIG. 22 is a continuation of FIG. 21 with the nucleotide sequence being SEQ ID NO: 33 and the amino acid sequence being SEQ ID NO: 34.

FIG. 23 shows SEQ ID NO: 35 an amino acid sequence (without including signal peptide) of AMP deaminase derived from Streptomyces which has been successfully identified.

FIG. 24 shows SEQ ID NO: 36 an amino acid sequence (including signal peptide) of AMP deaminase derived from Streptomyces which has been successfully identified.

FIG. 25 shows SEQ ID NO: 37 a sequence (including a promoter region and a terminator region) encoding AMP deaminase derived from Streptomyces which has been successfully identified.

FIG. 26 shows the continuation of FIG. 25 as SEQ ID NO: 38.

FIG. 27 shows SEQ ID NO: 39 a sequence of a promoter region of a gene encoding AMP deaminase derived from Streptomyces which has been successfully identified.

FIG. 28 shows SEQ ID NO: 40 a sequence of a structural gene of AMP deaminase derived from Streptomyces which has been successfully identified.

FIG. 29 shows SEQ ID NO: 41 a sequence of a terminator region of a gene encoding AMP deaminase derived from Streptomyces which has been successfully identified.

BEST MODE FOR CARRYING OUT THE INVENTION

The first aspect of the present invention relates to AMP deaminase. AMP deaminase of the present invention is derived from Streptomyces. The origin of AMP deaminase of the present invention is not limited to a particular species of Streptomyces. The present inventors have investigated and found that AMP deaminase produced by Streptomyces murinus, Streptomyces celluloflavus and Streptomyces griseus also has a high thermal resistant property.

AMP deaminase of the present invention catalyzes the following reaction: 5′-adenylic acid+H₂O→5′-inosinic acid+NH₃ (characteristic (1)). Thus, AMP deaminase of the present invention acts on 5′-adenylic acid (AMP). As shown in the below-mentioned Examples, AMP deaminase derived from Streptomyces murinus that is one embodiment of the present invention acts on 5′-dAMP (5′-deoxy adenylic acid), ADP (adenosine 5′-diphosphate) and ATP (adenosine 5′-triphosphate) favorably. Therefore, AMP deaminase of the present invention can be applied to not only a reaction using AMP as a substrate but also a reaction using 5′-dAMP, ADP and ATP as a substrate.

Meanwhile, AMP deaminase of the present invention is excellent in thermostability and stable at 65° C. or less (characteristic (2)). Herein, “stable at 65° C. or less” means that when an enzyme solution adjusted to pH 5.6 with an acetate buffer solution is treated at 65° C. for 30 minutes, 5% or more, preferably 10% or more, further preferably 30% or more, more further preferably 50% or more and the most preferably 90% or more of activity remains based on the reference (100%) in the case where the treatment is not carried out (the same treatment is carried out at temperatures of 65° C. or less, the residual activity is generally increased). Since excellent thermostability is provided, AMP deaminase of the present invention can act favorably at high temperatures (for example, 60° C., 65° C., and 70° C.).

As shown in the below-mentioned Examples, the present inventors have found an enzyme produced by Streptomyces murinus as one of AMP deaminases having the above-mentioned characteristics and have succeeded in purifying this enzyme. When the obtained AMP deaminase was investigated in detail, it was revealed that AMP deaminase has the following characteristics.

(3) The molecular weight by gel filtration is 48,000±2,000. Note here that the molecular weight by SDS-PAGE is 60,000±3,000.

(4) The optimum pH is around 5.6 (in McIlvaine buffer solution).

(5) The working pH is about 4.5 to about 8.5 (in McIlvaine buffer solution).

(6) The stable pH is about 6.0 to about 8.5 (in McIlvaine buffer solution).

(7) The reaction temperature is about 40° C. to 70° C. (in acetate buffer solution (pH 5.6)).

(8) The optimum temperature is around 65° C. (in acetate buffer solution (pH 5.6)).

(9) The temperature stability is stable at 65° C. or less (in acetate buffer solution (pH 5.6).

The range of the working pH is in the range in which the relative activity is about 50% or more when the AMP deaminase activity at the optimum pH is defined as reference (100%). Meanwhile, the range of the stable pH is in the range in which the residual activity is about 50% or more when the AMP deaminase activity at the optimum pH is defined as reference (100%).

As to the thermostability, when an enzyme solution that is adjusted to pH 5.6 with an acetate buffer solution is treated at 65° C. for 30 minutes, about 90% of residual activity is found with respect to a reference (100%) that is the enzyme activity when no treatment was carried out. When the treatment temperature was raised to 70° C., about 55% of activity was maintained.

Furthermore, the range of the reaction temperature is a range in which the relative activity is about 70% or more when the AMP deaminase activity at the optimum temperature is defined as reference (100%).

Meanwhile, when the substrate specificity was investigated, it was revealed that AMP deaminase derived from Streptomyces murinus acted on 3′-AMP, 5′-dAMP, ADP, ATP, Adenosine, and cAMP (cyclic adenosine-3′,5′-monophosphate) and that it did not act on 2′-AMP, adenine, 5′-GMP, 5′-UMP, and 5′-CMP. In particular, it was found that AMP deaminase acted on 5′-dAMP, ADP, and ATP favorably. The action on adenosine was weak, which was about 1/10 or less of the action on 5′-AMP.

Furthermore, a phosphatase activity was not detected. On the contrary, in conventional AMP deaminase derived from Aspergillus melleus, a contaminated phosphatase activity is detected. Thus, AMP deaminase derived from Streptomyces murinus is significantly different from conventional AMP deaminase in that the contaminated phosphatase activity is extremely small.

When phosphatase is contaminated, inosinic acid that is a taste component produced by AMP deaminase effect by 5′-AMP is further decomposed into inosine. As a result, a taste property is lost, so that AMP deaminase derived from Streptomyces murinus into which extremely little phosphatase is contaminated is industrially advantageous.

After all, AMP deaminase derived from Streptomyces murinus has the above-mentioned characteristics (3) to (8), thermostability, substrate specificity, and extremely small amount of contaminated phosphatase.

Note here that the AMP deaminase activity in various tests such as a thermostability test is measured based on methods described in the below-mentioned Examples.

The second aspect of the present invention relates to a production method (preparation method) of AMP deaminase and the method includes the following steps:

(a) culturing step of culturing Streptomyces of the genus Streptomyces.

(b) purification step of purifying AMP deaminase from a culture solution after the culturing step.

The kinds of Streptomyces used in the step (a) is not particularly limited as long as it is expected to produce AMP deaminase having excellent thermostability. For example, Streptomyces murinus, Streptomyces celluloflavus, or Streptomyces griseus can be used. Streptomyces can be cultured by a routine method. As a medium, a medium containing a carbon source such as glucose, sucrose, gentiobiose, soluble starch, glycerin, dextrin, syrup, organic acids, and the like, nitrogen source such as ammonium sulfate, ammonium carbonate, ammonium phosphate, ammonium acetate, or peptone, yeast extract, corn steep liquor, casein hydrolysate, bran, meat extract, and the like, and if necessary, inorganic chlorine (inorganic ion) such as potassium salt, magnesium salt, sodium salt, phosphate salt, manganese salt, iron salt, zinc salt, and the like, can be used. In order to promote the growth of Streptomyces, a medium to which vitamin, amino acid, and the like, have been added can be used. The pH of the medium is adjusted to, for example, in the range from 5.0 to 8.0, and preferably, in the range from 5.5 to 7.5. The culturing temperature is, for example, in the range from 15° C. to 50° C., preferably in the range from 20° C. to 40° C., and more preferably in the range from 25° C. to 35° C. The culturing time is not particularly limited and may be, for example, one day or more, three days or more, and five days or more. As a culturing method, for example, a shake culture method, and an aerobic submerged culture method with a jar fermenter can be employed.

The above-mentioned various culturing conditions may be changed appropriately depending upon the subjects to be cultured and the conditions are not particularly limited as long as AMP deaminase of the present invention can be produced.

From a culture solution or fungus body obtained after Streptomyces is cultured for a desired time, AMP deaminase can be collected. When AMP deaminase is collected from a culture solution, for example, a culture supernatant is subjected to filtration and centrifugation so as to remove insoluble matters, followed by isolation and purification by combining salting out such as ammonium sulfate precipitation, dialysis and various chromatography, and the like. Thereby, AMP deaminase can be obtained. Preferably, after fraction by salting out such as ammonium sulfate precipitation, hydrophobic chromatography and gel filtration are carried out.

On the other hand, when AMP deaminase is collected from a fungus body, for example, after the fungus body is crushed by pressurizing treatment, ultrasonic treatment, and the like, isolation and purification are carried out as mentioned above. Thereby, AMP deaminase can be obtained. Note here that the above-mentioned series of processes (crushing of fungus bodies, isolation, and purification) may be carried out after fungus bodies are collected from a culture solution in advance by filtration, centrifugation, and the like.

In each purification process, in principle, fraction is carried out by using an AMP deaminase activity as an index and then the following step is carried out.

The third aspect of the present invention relates to use of the above-mentioned AMP deaminase of the present invention. AMP deaminase of the present invention is used for various applications similar to conventional AMP deaminases (that is to say, AMP deaminase derived from Aspergillus melleus, which has been used to date). However, from the characteristic of having excellent thermal resistant property, AMP deaminase of the present invention can be suitably used in applications in which a reaction at a high temperature is preferred. Herein, “applications in which a reaction at a high temperature is preferred” means an application in which AMP deaminase is subjected to reaction at a high temperature from the viewpoint of production efficiency, contamination, and the like. A specific example of this application can include production of yeast extract. In the production of yeast extract, in general, taste is increased (taste is improved) by nuclease treatment and AMP deaminase treatment. When AMP deaminase of the present invention having excellent thermostability is used, AMP deaminase treatment can be carried out simultaneously with nuclease treatment that is carried out at a high temperature. This can simplify and shorten the production process and at the same time, can effectively prevent the contamination during the treatment of enzyme.

In the production method of yeast extract using AMP deaminase of the present invention, as a preferable embodiment, the next step, that is, a step of adding nuclease and AMP deaminase to yeast lysate and allowing them to act on yeast lysate at a high temperature is carried out. The yeast lysate can be prepared by a conventional method. For example, a suspension (pH 7.0) of 10% beer yeast, baker's yeast, or the like, is thermally treated and then commercially available lytic enzyme such as YL-NL “AMANO,” YL-15, or the like (both are products of Amano Enzyme Inc.) is added and lysed, so that yeast lysate is prepared. Nuclease is commercially available from various manufacturers (for example, Amano Enzyme Inc.) and suitable nuclease can be selected appropriately. Nuclease prepared by a conventional method from a microorganism may be used. Plural kinds of nucleases may be used combination. The addition amount of nuclease and AMP deaminase can be appropriately set taken the kinds or activity values of enzymes to be used into consideration. The working temperature is a temperature at which both nuclease and AMP deaminase can work. They are allowed to react at, for example, in the range from 60° C. to 80° C., preferably in the range from 65° C. to 75° C., and further more preferably 70° C.

(Isolated AMP Deaminase)

As shown in the below-mentioned Examples, the present inventors succeeded in identifying an amino acid sequence of AMP deaminase derived from Streptomyces murinus and a sequence of genes coding therefor. This makes it possible to produce AMP deaminase as a recombinant protein.

The further aspect of the present invention relates to an isolated AMP deaminase based on the above-mentioned achievement. AMP deaminase of the present invention includes, for example, a protein having an amino acid sequence set forth in SEQ ID NO: 1. As shown in the below-mentioned Examples, it is confirmed that the protein actually exhibits the AMP deaminase activity. Note here that a sequence including a signal peptide is shown in SEQ ID NO: 2.

One embodiment of the present invention provides AMP deaminase consisting of a protein having the same function as the protein having an amino acid sequence set forth in SEQ ID NO: 1 or 2 but having a difference in a part of the amino acid sequence (hereinafter, also referred to as “homologous protein”). The phrase “having a difference in a part of the amino acid sequence” typically means that the amino acid sequence includes mutation (change) in the amino acid sequence due to deletion, substitution of one to several of the amino acids constituting the amino acid sequence, or addition and insertion of one to several amino acids, or combination thereof. Herein, the difference in the amino acid sequence is acceptable as long as the function of catalyzing AMP deaminase, that is, a reaction: 5′-adenylic acid+H₂O→5′-inosinic acid+NH₃ (also referred to as “AMP deaminase activity”) is maintained. As long as this condition is satisfied, a position of difference in the amino acid sequence is not particularly limited and also may include differences in a plurality of positions. The plurality herein denotes a number corresponding to, for example, about less than about 30%, preferably less than about 20%, further preferably less than about 10%, further more preferably less than 5%, and most preferably less than about 1% with respect to total amino acids. That is to say, a homologous protein has the identity of about 70% or more, preferably about 80% or more, further preferably about 90% or more, further more preferably about 95% or more, and most preferably about 99% or more with respect to the amino acid sequence set forth in SEQ ID NO: 1 or 2.

Preferably, a homologous protein is obtained by allowing a conservative amino acid substitution to be generated in a nonessential amino acid residue (amino acid residue that is not related to “AMP deaminase activity”). Herein, a “conservative amino acid substitution” is one in which the amino acid residue is replaced with an amino acid residue having a side chain with similar feature. The amino acid residues are divided into some families including basic side chains (e.g., lysine, arginine, histidine), acidic side chains (e.g., aspartic acid, glutamic acid), uncharged polar side chains (e.g., glycine, asparagine, glutamine, serine, threonine, tyrosine, cysteine), nonpolar side chains (e.g., alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), beta-branched side chains (e.g., threonine, valine, isoleucine) and aromatic side chains (e.g., tyrosine, phenylalanine, tryptophan, histidine). Preferably, the conservative amino acid substitution is a substitution between preferably an amino acid residue of the same family.

The identity (%) of two amino acid sequences or of two nucleic acid sequences (hereinafter, referred to as “two sequences” as a term including these sequences) can be determined by, for example, the following procedures. Firstly, the two sequences are aligned so that two optimum comparison between two sequences can be conducted (e.g., gaps may be introduced in a first sequence for optimum alignment with second sequence). When the molecule (amino acid residue or nucleotide) at the certain position of the first sequence is the same as the molecule at corresponding position of the second sequence, then the molecules are identical at that position. The identity between the two sequences is a function of the number of identical positions shared by the sequences (i.e. identity (%)=number of identical positions/total number of positions×100). Preferably, the number of gaps, and the length of each gap, which need to be introduced for optimum alignment of the two sequences are considered.

The comparison of two sequences and determination of percent identity between two sequences can be accomplished using a mathematical algorithm. A specific example of the mathematical algorithm that can be used for comparison between sequences includes a mathematical algorithm that is described in Karlin and Altschul (1990) Proc. Natl. Acad. Sci. USA 87:2264-68 and that was modified by Karlin and Altschul (1993) Proc. Natl. Acad. Sci. USA 90:5873-77. However, the mathematical algorithm is not particularly limited to this. Such an algorithm is incorporated into NBLAST program and XBLAST program (version 2.0). In order to obtain a nucleotide sequence being homologous to the nucleic acid of the present invention, for example, with NBLAST program, BLASTnucleotide search is carried out with score=100 and wordlength=12. In order to obtain an amino acid sequence being homologous to the protein of the present invention, for example, with XBLAST program, BLASTpolypeptide search is carried out with score=50 and wordlength=3. In order to obtain a gap alignment for comparison, Gapped BLAST described in Altschul et al. (1997) Amino Acids Research 25 (17): 3389-3402 can be used. In the case where BLAST and Gapped BLAST are used, default parameter of the corresponding program (for example, XBLAST and NBLAST) can be used. Another example of a mathematical algorithm that can be used for comparison of sequence includes an algorithm described in Myers and Miller (1988) Comput Appl Biosci. 4:11-17. Such an algorithm is incorporated into an ALIGN program that can be used in, for example, a GENESTREAM network server (IGH Montpellier, France) or an ISREC server. In the case where the ALIGN program is used for comparison of the amino acid sequence, for example, PAM120 residue mass table is used, and gap length penalty can be made to be 12 and gap penalty can be made to be 4.

The identity between two amino acid sequences can be determined using the GAP program in the GCG software package using either a Blossum 62 matrix or a PAM250 matrix with the gap weight of 12, 10, 8, 6, or 4 and the length weight of 2, 3, or 4. Furthermore, the homology between two nucleic acid sequences can be determined using the GAP program in the GCG software package with a gap weight of 50 and a length weight of 3.

In AMP deaminases of the present invention (including homologous protein), AMP deaminase of natural Streptomyces can be prepared from the Streptomyces by the operations such as extraction, purification, and the like. Furthermore, by using a genetic engineering technique based on this specification and sequence information disclosed in an attached sequence list, AMP deaminase of the present invention (including homologous protein) can be prepared. For example, AMP deaminase of the present invention can be prepared by transforming an appropriate host cell by DNA encoding AMP deaminase and collecting protein expressed by the transformant. The collected protein can be appropriately purified in accordance with the purpose. In the case where AMP deaminase is prepared as recombinant protein, various modifications can be carried out. For example, DNA encoding AMP deaminase of the present invention and other appropriate DNA are inserted into the same vector and the vector is used for producing recombinant protein. Then, a recombinant protein in which other peptide or protein is linked to AMP deaminase of the present invention can be obtained. Furthermore, modification such as addition of sugar chain and/or lipid or processing of N-terminus or C-terminus may be carried out. The above-mentioned modification enables extraction of recombinant protein, simplification of purification, addition of biological functions, or the like.

Herein, “isolated” used with respect to AMP deaminase of the present invention denotes that AMP deaminase exists in a state in which they are taken out from its original environment (for example, natural environment when AMP deaminase is a natural material), that is, in a state in which they are different form from the original form.

Isolated AMP deaminase generally does not contain a fungus body component of a producing strain. Furthermore, it is preferable that the content of the contaminated components (contaminated protein, other components derived from the host when it is produced by a recombination DNA technology, component of a culture solution, and the like) is small. The amount of the contaminated protein in the isolated AMP deaminase of the present invention is, for example 50% or less, preferably 40% or less, further preferably 30% or less, and more preferably 20% or less of the total amount of the protein.

(Nucleic Acid Molecule Encoding AMP Deaminase)

The further aspect of the present invention relates to a nucleic acid molecule encoding AMP deaminase.

The term “nucleic acid” in the present invention includes DNA (including cDNA and genome DNA), RNA (including mRNA), DNA analogs and RNA analogs. The form of the nucleic acid of the present invention is not particularly limited. That is to say, the nucleic acid may be any of a single strand and a double strand. However, a double-stranded DNA is preferable. The degeneration of codon is also taken into consideration. That is to say, the nucleic acid may have any nucleotide sequences as long as the target protein can be obtained as an expressed product.

The term “nucleic acid encoding a specific protein” denotes nucleic acid from which the protein is obtained when it is expressed. It includes not only nucleic acid having a nucleotide sequence corresponding to an amino acid sequence of the protein but also nucleic acid to which a sequence that does not encode an nucleic acid is added to the above-mentioned nucleic acid (for example, DNA containing one or a plurality of introns).

The term “isolated nucleic acid molecule” in this specification includes nucleic acid molecules that are separated from other nucleic acid molecules coexisting in a natural state of the nucleic acid. However, a part of other nucleic acid components such as a nucleic acid sequence neighboring in the natural state may be contained.

In the case of nucleic acid produced by genetic recombination technology, for example, cDNA molecule, the “isolated nucleic acid” denotes nucleic acid in a state in which cell components, culture solution, and the like, are not substantially contained. Similarly, in the case of nucleic acid produced by chemical synthesis, the “isolated nucleic acid” denotes nucleic acid in a state in which a precursor such as dNTP or chemical materials used in the synthesis process is not substantially contained.

Whether nucleic acid is present as a part of a vector or a composition, or nucleic acid is present in a cell as a foreign molecule, the nucleic acid is “isolated nucleic acid” as long as it is present as a result of artificial operation.

Unless otherwise noted, when merely the term “nucleic acid” is used in this specification, it signifies that nucleic acid in a state in which it isolated.

The nucleic acid molecule of the present invention encodes above-mentioned AMP deaminase of the present invention. That is to say, the nucleic acid molecule of the present invention encodes a protein having an amino acid sequence set forth in SEQ ID NO: 1 or 2 or the homologous protein thereof. The specific embodiment of the nucleic acid molecule of the present invention is DNA having a nucleotide sequences of SEQ ID NO: 3. This nucleotide sequence is DNA encoding a successfully identified AMP deaminase gene and includes 5′ non-translation region (promoter region) and 3′ non-translation region (terminator region). Such a DNA includes a original combination (combination in natural state) of promoter, terminator and structural gene. Therefore, when the DNA is used for producing AMP deaminase, excellent gene expression is expected. Therefore, an efficient AMP deaminase production system can be constructed.

Another embodiment of the present invention provides DNA in which any one or more of 5′ non-translation region or a part thereof, and 3′ non-translation region or a part thereof are deleted from the nucleotide sequences of SEQ ID NO: 3. Specific examples of such DNA can include DNA having a nucleotide sequences of SEQ ID NO: 4 or 5. DNA having the nucleotide sequences of SEQ ID NO: 4 is DNA encoding a structural gene (including a region encoding signal peptide) of successfully identified AMP deaminase. Similarly, DNA having the nucleotide sequences of SEQ ID NO: 5 is DNA encoding a structural gene (which does not include a region encoding signal peptide) of successfully identified AMP deaminase.

The nucleic acid of the present invention can be prepared in an isolated state by standard genetic engineering technique, molecular biological technique, biochemical technique, and the like, with reference to sequence information disclosed in this specification or attached sequence list.

For example, the nucleic acid of the present invention can be isolated from Streptomyces genome DNA library by using a hybridization method using all or a part of the nucleotide sequence of the nucleic acid or the complementary sequence thereof as a probe. Furthermore, by using a nucleic acid amplification reaction (for example, PCR) using a synthesized oligonucleotide primer designed to specifically hybridize a part of a nucleotide sequence of the nucleic acid, the nucleic acid of the present invention can be amplified and isolated from Streptomyces genome DNA library or Streptomyces genome or Streptomyces nucleic acid extract. In general, an oligonucleotide primer can be easily synthesized by using, for example, an automated DNA synthesizer.

In accordance with kinds of libraries to be used, a plaque hybridization method, a colony hybridization method, or the like can be used (see, for example, Molecular Cloning, Third Edition, Cold Spring Harbor Laboratory Press, New York). For example, in the case of a library constructed by using, for example, a plasmid, a colony hybridization method is employed. For selection of a clone having the target nucleic acid, a probe having a sequence specific to the nucleic acid of the present invention is used. When the target clone is selected, by carrying out PCR method and the like using a primer specific to the sequence of the target nucleic acid by using a nucleic acid carried by this clone as a template, the nucleic acid of the present invention can be obtained as an amplified product.

The nucleic acid carried by the obtained clone can be subcloned in an appropriate vector for use in use later. Thus, for example, it is possible to construct a recombinant vector for transformation or a plasmid suitable for decoding a nucleotide sequence.

Another embodiment of the present invention provides nucleic acid that has a nucleotide sequence having the same function as the nucleotide sequences of any of SEQ ID NOs: 3 to 5 but having a difference in a part of the nucleotide sequence (hereinafter, referred to as “homologous nucleic acid”). An example of the homologous nucleic acid can include DNA encoding a protein including a nucleotide sequence in which one or a plurality of nucleotides are substituted, deleted, inserted, added or inverted relative to the nucleotide sequences of SEQ ID NOs: 3 to 5 and having an AMP deaminase activity. Such substitution, deletion, or the like, may be occurred in a plurality of sites. The “plurality” herein differs depending upon the position or kinds of amino acid residues in a three-dimensional structure of a protein encoded by the nucleic acid codes, but the “plurality” of nucleotides includes, for example, 2 to 40 nucleotides, preferably 2 to 20 nucleotides and more preferably 2 to 10 nucleotides.

The above-mentioned mutation such as substitution, deletion, insertion, addition or inversion of nucleotide includes a naturally occurring mutation, for example, difference in individual, species, or genus of microorganism carrying an AMP deaminase gene.

Another example of the homologous nucleic acid can include a nucleic acid having difference in nucleotide as mentioned above due to polymorphism represented by SNP.

The above-mentioned homologous nucleic acid can be obtained by, for example, a treatment with a restriction enzyme; treatment with exonuclease, DNA ligase, etc; introduction of mutation by a site-directed mutagenesis (Molecular Cloning, Third Edition, Chapter 13, Cold Spring Harbor Laboratory Press, New York); random mutagenesis (Molecular Cloning, Third Edition, Chapter 13, Cold Spring Harbor Laboratory Press, New York), and the like. Furthermore, the homologous nucleic acid can be also obtained by the other method such as ultraviolet irradiation. In addition, the homologous nucleic acid can be also obtained by a well-known method using a mutation treatment, for example, treating Streptomyces carrying an AMP deaminase gene with ultraviolet ray, followed by isolating the modified gene.

A specific example of a method for preparing the homologous nucleic acid will be described hereinafter. The method includes: extracting a genome (chromosomal) DNA from naturally occurring Streptomyces carrying a homologous nucleic acid; treating the extracted DNA with an appropriate restriction enzyme; and then selecting and isolating a DNA that hybridizes under stringent conditions in a screening using the nucleic acid molecule of the present invention (for example, DNA having a sequence of SEQ ID NO: 3) or a part thereof as a probe. In the case where a genome (chromosomal) DNA library including a clone carrying a modified DNA can be used, the DNA can be obtained by screening the library using the nucleic acid molecule of the present invention (for example, DNA having a sequence of SEQ ID NO: 3) or a part thereof as a probe under stringent conditions.

Another embodiment of the present invention relates to a nucleic acid having a nucleotide sequence complementary to the nucleotide sequences of SEQ ID NOs: 3 to 5.

A further embodiment of the present invention provides nucleic acid having a nucleotide sequence at least 60%, 70%, 80%, 90%, 95%, 99% or 99.9% identical to the nucleotide sequences set forth in SEQ ID NOs: 3 to 5 or a nucleotide sequence complementary to any of them. The identity is preferred to be as high as possible.

A further embodiment of the present invention relates to nucleic acid having a nucleotide sequence that hybridizes, under stringent conditions, to the nucleotide sequence complementary to the nucleotide sequences of SEQ ID NOs: 3 to 5 or the homologous nucleotide sequence thereof. The “stringent conditions” herein denote a condition in which a so-called specific hybrid is formed and a non-specific hybrid is not formed. Such stringent conditions are well known to the person skilled in the art and can be set with reference to Molecular Cloning (Third Edition, Cold Spring Harbor Laboratory Press, New York) or Current protocols in molecular biology (edited by Frederick M. Ausubel et al., 1987). An example of the stringent conditions includes a condition in which a DNA is incubated in a hybridization solution (50% formamide, 10×SSC (0.15 M NaCl, 15 mM sodium citrate, pH 7.0), 5×Denhardt solution, 1% SDS, 10% dextran sulfate, 10 μg/ml denatured salmon sperm DNA, 50 mM phosphate buffer (pH7.5)) at about 42° C. to about 50° C., followed by washing with 0.1×SSC and 0.1% SDS at about 65° C. to about 70° C. A more preferable example of the stringent conditions can include a condition using a hybridization solution (50% formamide, 5×SSC (0.15 M NaCl, 15 mM sodium citrate, pH 7.0), 1×Denhardt solution, 1% SDS, 10% dextran sulfate, 10 μg/ml denatured salmon sperm DNA, 50 mM phosphate buffer (pH 7.5)).

(Vector)

Another aspect of the present invention relates to a vector containing the nucleic acid of the present invention. The term “vector” in this specification refers to a nucleic acid molecule that can transport a nucleic acid inserted therein to a cell, and the like.

The vector of the present invention can be prepared by introducing the nucleic acid of the present invention (typically, DNA) into an existing vector or a vector obtained by modifying the existing vector. Any vectors may be used as a starting material in principle as long as they can carry the nucleic acid of the present invention, however, an appropriate vector can be selected in accordance with the purpose of use (cloning, expression of polypeptide), and considering the kinds of host cells.

Typically, a vector for transformation contains an AMP deaminase gene (for example, DNA having a sequence of SEQ ID NO: 4), a promoter and a terminator. In order to achieve an appropriate transcription of a structural gene by a promoter, a promoter, an AMP deaminase gene and a terminator are arranged successively from the upper stream toward the lower stream.

The vector of the present invention is preferably an expression vector. The “expression vector” is a vector capable of introducing the nucleic acid inserted into the vector to the inside of the cell (host cell) and capable of expressing in the cell. The expression vector generally includes a promoter sequence necessary for expression of nucleic acid, an enhancer sequence promoting the expression, and the like. An expression vector including a selected marker can be used. In the case where such an expression vector is used, it is possible to confirm the presence (and the extent) of introduction of the expression vector by using the selected marker.

An insertion of the nucleic acid of the present invention into a vector, an insertion of a selected marker gene (if necessary), insertion of a promoter (if necessary), and the like, can be carried out by using a standard recombination DNA technology (for example, Molecular Cloning, Third Edition, 1.84, Cold Spring Harbor Laboratory Press, New York can be referred. A well-known method restriction enzyme and DNA ligase). Note here that when a vector carrying DNA that also includes a promoter region (for example, DNA having a nucleotide sequences of SEQ ID NO: 3) is constructed, the recombinant vector may be constructed by preparing the promoter region of DNA and the other region independently and introducing them into a vector. In such a case, on the condition that a promoter function can be appropriately exhibited, other sequence may be intervened between both regions (promoter region and the other regions) in the vector. Furthermore, firstly, a vector carrying a promoter region may be constructed and then linked to the other regions.

The above-mentioned recombinant vector is used for transformation of a host. That is to say, by using the above-mentioned recombinant vector, transformant into which the nucleic acid molecule of the present invention is introduced can be prepared.

(Transformant and Use thereof)

The vector for transformation can be used for transforming a host. That is to say, by using the above-mentioned vector for transformation, a transformant into which the nucleic acid molecule of the present invention has been introduced can be prepared.

The host used for transformation is not particularly limited. Streptomyces such as Streptomyces murinus (IFO 14802, etc.), Streptomyces lividans (TK24, etc.), Streptomyces griseus subsp. griseus, Streptomyces griseus, Streptomyces celluloflavus, Streptomyces coelicolor, Streptomyces mobaraensis, Streptomyces avermitilis can be preferably used as a host. In addition to the above, Escherichia coli, Bacillus subtilis, Saccharomyces cerevisiae, Saccharomyces pombe, and the like, can be employed as a host.

The introduction of the vector for transformation into a host (transformation) can be carried out by a well-known method. For example, the transformation can be carried out by a method by D. A. Hopwood et al. (PRACTICAL STREPTOMYCES GENETICS P. 229-252 (The John Innes Foundation, 2000)) using protoplast fungus body.

The transformant can be used for production of AMP deaminase. Specifically, the transformant into which the nucleic acid of the present invention has been introduced is cultured under conditions capable of allowing protein (AMP deaminase) coded by the nucleic acid to express, and thereby AMP deaminase can be produced. The medium can be appropriately used in accordance with a host to be used. For example, it is possible to use commercially available various media or media to which proline, leucine, thiamin, and the like, which are components necessary for growth of transformant, selection, and promoting the expression of protein are added.

From a culture solution or fungus body, which have been cultured for a predetermined time, a target protein (AMP deaminase) can be collected. In the case where the proteins are produced outside the fungus body, they can be collected from the culture solution and in the other cases, the proteins can be collected from the inside of the fungus body. When the proteins are collected from a culture solution, for example, they can be obtained by subjecting the culture supernatant to filtration, centrifugation so as to remove insoluble substances, followed by carrying out isolation and purification by combining salting out such as ammonium sulfate precipitation, dialysis, various chromatography, and the like. On the other hand, when the proteins are collected from the inside of the fungus body, for example, they can be obtained by crushing the fungus body by pressurizing treatment, ultrasonic treatment, followed by carrying out isolation and purification as mentioned above. Note here that the above-mentioned series of processes (crushing of fungus bodies, isolation, and purification) may be carried out after fungus bodies are collected from a culture solution in advance by filtration, centrifugation, and the like. Since AMP deaminase of the present invention is produced outside of the fungus body, isolation and purification is carried out relatively easily.

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not necessarily limited to these Examples.

Example 1 Purification of AMP Deaminase Derived from Streptomyces murinus

1. Materials and Methods

Strains to be used in the following experiments, a preparation method of an enzyme solution and a method for measuring the enzyme activity are as follows.

<Strains to be Used>

Streptomyces murinus IFO14802, Streptomyces griseus subsp. griseus JCM4681, Streptomyces griseus IFO3355 (NBRC3355), and Streptomyces celluloflavus IFO13780 (NBRC13780) are used. Note here that Streptomyces murinus IFO14802 is deposited with the following international depositary agency. International depositary agency

Name: National Institute of Advanced Industrial Science and Technology, International Patent Organism Depositary

Address: Chuo No. 6, 1-3, Higashi 1-chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan

Deposition date: Mar. 29, 2004

Deposition (accession) No.: FERM BP-08673

<Method of Preparing Enzyme Solution>

(1) Enzyme Solution Derived from Aspergillus melleus

“Deamizyme (50,000 u/mg product) (trade name)” that is a commercially available AMP deaminase agent from Amano Enzyme Inc. is diluted with water so as to obtain an enzyme solution.

(2) Enzyme Solution Derived from Streptomyces murinus

Solpee NY (2%), Meast P1G (0.5%), NaCl (0.3%), KH₂PO₄ (0.1%), food additive MgSO₄ (0.05%) and soluble starch (3%) were added so as to adjust to pH 5.7 and sterilized at 121° C. for 30 minutes. Streptomyces murinus IFO14802 was inoculated, and precultured for one day, main-cultured for 5 days at 27° C. so as to prepare a crude enzyme solution.

(3) Enzyme Solution Derived from Streptomyces

Soyaflour A (2%), NaCl (0.3%), KH₂PO₄ (0.1%), food addition MgSO₄ (0.05%) and soluble starch (3%) were added so as to adjust to pH 5.7 and sterilized at 121° C. for 30 minutes, which is inoculated, precultured for one day and main-cultured for 5 days at 27° C. Thus, a crude enzyme solution was prepared.

<Method of Measuring Enzyme Activity>

(1) Method of Measuring AMP Deaminase Activity

By using the decrease of OD₂₆₅ at the time of reaction as an index, the enzyme activity was measured. To 1.5 ml of solution obtained by mixing 0.017M 5′ AMP-2Na and 1/15M phosphate buffer solution (pH 5.6) at the ratio of 1:2, 0.5 ml of sample solution was added so as to obtain a reaction solution, which was reacted at 37° C. for 15 minutes. Then, 2% perchloric acid solution was added so as to stop the reaction and 100 μl of the solution was taken out. Water was added to the solution so that the total amount was 5 ml. Then, OD₂₆₅ was measured. The value similarly measured at reaction time of 0 minute was defined as a blank. Under the above-mentioned conditions, a case where an absorbance difference is reduced by 0.001 during 60 minutes was defined as one unit.

(2) Method of Measuring Phosphatase Activity

To 1.5 ml of solution obtained by mixing 0.025M 5′IMP-2Na and 0.036M barbital sodium-hydrochloric acid buffer solution (pH 5.6) at the ratio of 1:2, 0.5 ml of sample solution was added so as to obtain a reaction solution, which was reacted at 37° C. for 30 minutes. After the reaction was completed, 200 μl of the solution was taken out, and 5 ml of 6% perchloric acid solution was added so as to stop the reaction. Amidol solution (0.05M, 0.25 ml) was added and mixed, followed by adding and mixing 0.25 ml of 0.067 M ammonium molybdate solution, and further adding and mixing 0.25 ml of water, which was allowed to stand in flowing water for 15 minutes. Then, OD₂₆₅ was measured. A value similarly measured at the reaction time of 0 minute was defined as a blank. Under the above-mentioned conditions, a case where an absorbance difference is reduced by 0.001 during 30 minutes was defined as one unit.

2. Results

(1) Theremostability

Thermostability was compared between AMP deaminase produced by Aspergillus melleus and AMP deaminase produced by Streptomyces murinus. Aspergillus melleus and Streptomyces murinus were examined for the residual AMP deaminase activity (pH 5.6) after 1% enzyme solution prepared by the above-mentioned method was treated at a predetermined temperature. Note here that the treating temperatures were set to 30° C., 40° C., 50° C., 60° C., 65° C., 70° C., and 75° C. The treating time was set to 30 minutes.

The measurement results are shown in a graph of FIG. 1. FIG. 1 shows that AMP deaminase derived from Streptomyces murinus was extremely excellent in thermostability.

(2) Contaminated Phosphatase

The contaminated phosphatase activities concerning Aspergillus melleus and Streptomyces murinus were measured. The phosphatase activity/deaminase activity (P/D) calculated from the measurement results are summarized in FIG. 2. In Aspergillus melleus, the contamination activity was observed. On the other hand, in Streptomyces murinus, the phosphatase activity was not observed. That is to say, it was found that AMP deaminase produced by Streptomyces murinus had extremely low contaminated phosphatase activity.

(3) IMP Conversion Reaction

IMP conversion rate by HPLC of AMP deaminase produced by Aspergillus melleus and Streptomyces murinus was examined. Specifically, by using 1/15M phosphate buffer solution (pH 7.0), 1.1% AMP solution was prepared. To 5 ml of 1.1% AMP solution, 0.5 ml of enzyme solution was added and reacted at 50° C. for 4 hours. Thereafter, heat treatment at 100° C. was carried out for 10 minutes, filter filtration (0.45 μm) was carried out, and HPLC analysis was carried out.

The measurement results are shown in FIG. 3. The IMP conversion rate (IMP conversion speed) of AMP deaminase produced by Streptomyces murinus was the same level as that of AMP deaminase produced by Aspergillus melleus. As mentioned above, since in Streptomyces murinus did not exhibit contaminated phosphatase activity, specification of the side product was carried out. As a result, the ratio of the side product (hypoxanthine) was smaller than the case of Aspergillus melleus (results are not shown).

(4) Feasibility Test

In order to confirm that AMP deaminase derived from Streptomyces murinus is effective in the actual production of yeast extract, the following tests (test 1 and test 2) were carried out.

a) Test 1

In test 1, similar to the current production method, treatment with nuclease and treatment with deaminase were carried out independently. Specifically, the treatment was carried out by the following procedure and the IMP conversion rate was calculated. Firstly, nuclease “AMANO” G (Amano Enzyme Inc., 0.1% w/w yeast solid) was added to yeast lysate (YL-15, 0.2%) and they were reacted with each other under the conditions at a temperature of 70° C. and pH 5 for 3 hours. Then, “Deamizyme (50,000 u/mg product) (trade name of Amano Enzyme Inc., 0.01 to 0.04% w/w yeast solid)” that is an AMP deaminase agent derived from the test enzyme (Aspergillus melleus), or a crude enzyme solution prepared from Streptomyces murinus was added and reacted with each other under the conditions at a temperature of 50° C. and pH 6 for 5 hours. Then, the reacted product was heat treated, and then subjected to HPLC analysis. Note here that added amount of AMP deaminase derived from Streptomyces murinus was set so that the activity value thereof became the same as that of “Deamizyme (50,000 u/mg product)” (trade name, Amano Enzyme Inc.)

b) Test 2

In test 2, treatment with nuclease and treatment with deaminase were carried out simultaneously. Specifically, the treatment was carried out by the following procedure and the IMP conversion rate was calculated. Firstly, nuclease “AMANO” G (Amano Enzyme Inc., 0.1% w/w yeast solid) and “Deamizyme (50,000 units) (trade name of Amano Enzyme Inc., 0.01 to 0.04% w/w yeast solid)” that is an AMP deaminase agent derived from the test enzyme (Aspergillus melleus) or a crude enzyme solution prepared by using Streptomyces murinus were added to yeast lysate (YL-15, 0.2%) and they were reacted with yeast lysate under the conditions at a temperature of 70° C. and pH5 for a predetermined time (3 hours or 5 hours). Then, the reacted product was heat treated, and then subjected to HPLC analysis. Note here that added amount of AMP deaminase derived from Streptomyces murinus was set so that the activity value thereof became the same as that of “Deamizyme (50,000 u/mg product)” (trade name, Amano Enzyme Inc.)

The results of test 1 are shown in FIG. 4 (a). As is apparent from FIG. 4 (a), AMP deaminase from Streptomyces murinus exhibits the equivalent IMP conversion rate to that of AMP deaminase derived from Aspergillus melleus (Deamzyme: 50,000 u/mg product).

On the other hand, the results of test 2 are shown in FIG. 4 (b). In this graph, “3 hrs” denotes the measurement results when AMP deaminase derived from Streptomyces murinus was allowed to act on simultaneously for 3 hours; and “5 hrs” denotes the measurement results when AMP deaminase derived from Streptomyces murinus was allowed to act simultaneously for 5 hours. Note here that no IMP was produced in the case where AMP deaminase derived from Aspergillus melleus (Deamzyme; 50,000 u/mg, product) was used (both in the reaction for 3 hours and 5 hours). As is apparent from FIG. 4 (b), when AMP deaminase derived from Streptomyces murinus was used, IMP was produced when it was allowed to act simultaneously with nuclease at high temperatures. Thus, it was confirmed that AMP deaminase derived from Streptomyces murinus was able to be reacted simultaneously with nuclease.

(5) Enzymological Property of AMP Deaminase Derived from Streptomyces murinus

AMP deaminase derived from Streptomyces murinus has been attempted to be purified by the following procedures. Firstly, Streptomyces murinus (IFO14802) was cultured by the above-mentioned method and the produced enzyme was concentrated two times with ultrafiltration membrane (AIP1010). Water was added to the concentrated solution, which was further concentrated so as to remove low molecular fractions. Then, the product was lyophilized so as to obtain a crude enzyme. This crude enzyme was dissolved in purified water and subjected to ammonium sulfate fractionation by using saturated ammonium sulfate (48%). The precipitated fraction was dissolved in 20 mM KPB (pH 7.0) so as to obtain an enzyme solution. The obtained enzyme solution was allowed to pass through HiPrep™16/10 ButylFF (Pharmacia Corporation) that had been equilibrated with 20 mM KPB (pH 7.0) containing 30% ammonium sulfate, followed by eluting at a concentration gradient of 300%˜0% by 20 mM KPB (pH 7.0) containing ammonium sulfate (30%˜0%). Thereafter, they were concentrated and the active fraction was subjected to gel filtration by using Superose 12 column (Pharmacia Corporation) so as to be eluted with 50 mM KPB (pH 7.0) containing 150 mM NaCl. Active fractions (fraction 15 and 16) were collected so as to obtain purified enzyme. Note here that the results of chromatography by using HiPrep™16/10 ButylFF is shown in FIG. 5( a); and the results of chromatography by using Superose 12 is shown in FIG. 5( b). Furthermore, the total activity amount, the total protein amount, specific activity, and yield in each stage are shown in FIG. 6 (a). The specific activity at the final stage came to be 96 times as that of the crude enzyme.

Purified enzyme was subjected to SDS-PAGE (CBB staining), and the purity of protein was confirmed. The results of SDS-PAGE are show in FIG. 6 (b). Lane II shows a sample lane (purified enzyme). Lane II shows a single band, showing that the purity of the purified enzyme is high. Note here that lane I shows bands of protein molecular weight markers. Bands of phosphorylase b (M.W. 97,400), bovine serum albumin (M.W. 66,267), aldolase (M.W. 42,400), carbonic anhydrase (M.W. 30,000), trypsin inhibitor (M.W. 20,100), and lysozyme (M.W. 14,400) are shown sequentially from the side of high molecular weight.

The relation between the reaction temperature and activity of the obtained purified enzyme was examined. The measurement results are shown in FIG. 7( a). A graph of FIG. 7 (a) shows the relative activity (%) when the activity value in the reaction at 65° C. is defined as 100%. As is apparent from the graph of FIG. 7 (a), high activity is observed in the wide range from 40° C. to 70° C. Furthermore, even in the case where the reaction temperature is 75° C., about 50% activity is found. Thus, it is shown that the enzyme acts favorably under the wide range of temperatures.

Subsequently, the thermostability of the enzyme was examined by the following procedure. An enzyme solution (0.15 ml) containing 2.8 μg protein (total amount) was adjusted to pH 5.6 by using acetate buffer (pH 5.0), and then treated for 30 minutes at each temperature (40° C., 50° C., 60° C., 65° C., 70° C., and 75° C.). Then, the residual activity (percent with respect to the enzyme activity in the case where the treatment was not carried out) was measured. The measurement results are shown in FIG. 7 (b). At 65° C. or less, about 90% or more of activity is maintained. Furthermore, also when the treatment is carried out at 70° C., about 55% of the activity is maintained. Thus, it was found that the enzyme had extremely excellent thermostability.

Next, the relation between pH and activity of the obtained purified enzyme was examined. The measurement results are shown in FIG. 8( a). A graph of FIG. 8 (a) shows the relative activity (%) when the activity value obtained in the reaction at pH 5.6 is defined as 100%. In the wide range of pH from about 4.5 to about 8.5, relatively high activity is observed. In the range of pH from about 5.0 to about 8.0, 70% or more of the reaction property is observed. Furthermore, it is shown that even in the case where pH is 9.0, about 40% of reactivity is obtained.

Next, pH stability of the enzyme was examined by the following procedure. An enzyme solution (0.15 ml) containing 2.8 μg protein (total amount) was adjusted to pH 3 to 8 by using McIlvaine buffer solution, and to pH 8 to 10 by using Atkins-pantin buffer solution. The solution was then allowed to stand at 30° C. for 30 minutes. Thereafter, the residual activity (percent with respect to the enzyme activity in the case where the treatment was carried out in pH 7.0) was measured. The measurement results are shown in FIG. 8 (b). It is shown that in the range of pH from about 6.0 to about 8.5, relatively high activity is maintained, and in the range of pH from about 6.5 to about 8.0, about 80% or more of activity is maintained.

(6) Substrate Specificity of AMP Deaminase Derived from Streptomyces murinus

It is reported that adenosine deaminase derived from Streptomyces aureofaciens has a wide range of substrate property, for example, catalytic ability of AMP (non-patent document 2). Taken this into consideration, in order to confirm whether deaminase derived from Streptomyces murinus obtained in the above-mentioned method is AMP-deaminase or adenosine-deaminase, the substrate specificity was investigated. The results are shown in FIG. 9. Note here that the relative activity, when the enzyme activity with respect to 5′ AMP is defined as 100% is represented. The enzyme acted on 5′ AMP most favorably. Furthermore, the enzyme also acted on 3′AMP, 5′dAMP, ADP, ATP, Adenosine and 3′5′-cyclic AMP, but the enzyme did not act on 2′AMP, adenine at all. In particular, the enzyme acted on 5′dAMP, ADP and ATP favorably. From the above-mentioned results, it was confirmed that the deaminase derived from Streptomyces murinus obtained by the above-mentioned method was AMP deaminase. Furthermore, it was determined that the enzyme was able to be employed preferably in a reaction using 5′dAMP, ADP and ATP as a substrate.

(7) Enzymological Property of AMP Deaminase Derived from Streptomyces murinus

In order to obtain an isoelectric point of AMP deaminase derived from Streptomyces murinus obtained by the above-mentioned method, chromatofocusing was carried out. By using MonoP column (Pharmacia), a starting buffer (Start Buffer) and an elution buffer (Poly buffer 96) were adjusted to pH 4.0 with 0.075M Tris acetate buffer (pH9.3) and with hydrochloric acid, respectively. The final amount was made to 100 ml. Furthermore, Pre-gradient with 3 ml, gradient elution with 30 ml and elution with 30 ml were carried out. The results of the chromatofocusing is shown in FIG. 10. From the results, it was thought that at least two isozymes were present (pI 8.12, 7.02), but the isoelectric point of main fraction was 8.12. Various properties, including the isoelectric point obtained from the above-mentioned results, of AMP deaminase derived from Streptomyces murinus, are shown in FIG. 11. As shown in this table, AMP deaminase derived from Streptomyces murinus had a molecular weight of about 48,000±2,000 in gel filtration (while molecular weight was 60,000±3,000 in SDS-PAGE). The optimum pH was around 5.6, the optimum temperature was about 65° C., the isoelectric point was 8.12, Km value was 0.95 mM and Vmax was 3.5×10⁷ μmol/min/mg.

(8) Search of AMP Deaminase Producing Strain from the Genus Streptomyces

Since the above-mentioned investigation showed that Streptomyces murinus, which was one of the Streptomyces of the genus Streptomyces, produced thermal resistant AMP deaminase, it was expected that thermal resistant AMP deaminase might be produced from the other strain of the genus Streptomyces. Then, in the genus Streptomyces in the stored strains of Amano Enzyme Inc., screening of AMP deaminase producing strain was carried out. As a result, a strain capable of producing AMP deaminase was found in three strains, that is, Streptomyces griseus subsp. griseus, Streptomyces griseus, and Streptomyces celluloflavus. The thermostability and substrate specificity of AMP deaminase produced form these three strains were investigated. The results are shown in FIG. 12. In the thermostability test, a crude enzyme solution obtained by culture was used, and the treatment was carried out under the conditions at 65° C. for 30 minutes. Furthermore, the residual activity was measured when the activity obtained when treatment was not carried out was defined as 100%. With respect to the substrate specificity (specificity with respect to adenosine), the relative activity with respect to the activity value when AMP was used as a substrate was defined as 100%.

As is apparent from FIG. 12, the above-mentioned three strains had more excellent thermostability than deaminase derived from Aspergillus melleus did. This suggested that deaminase produced by Streptomyces of the genus Streptomyces generally tended to have high thermostability. Note here that it was confirmed that deaminase produced by the above-mentioned three strains acted on AMP more favorably than adenosine did.

Example 2 Identification of AMP Deaminase Derived from Streptomyces murinus

1. Analysis of Amino Acid Sequence

Streptomyces murinus IFO 14802 was cultured in accordance with the procedures in the above-mentioned Examples so as to obtain a purified enzyme. However, purification was stopped at the purification by Butyl Sepharose column by using HiPrep™16/10 ButylFF (Pharmacia). Fraction samples were subjected to SDS-PAGE by using gel PAG Mini “DAIICHI” 10/20 (Daiichi Pure Chemicals Co., Ltd.). Gel after electrophoresis was transferred to a PVDF membrane by using Towbin buffer solution (25 mM Tris, 192 mM glycine, and 5% methanol) containing 0.01% SDS as a buffer solution. The transferring operation was carried out by using a buffer tank type transferring device under the conditions at constant voltage of 20V, at a temperature of 4° C. for 18 hours. After transfer, CBB (Coomassie Brilliant Blue R 250) (Fluka) staining was carried out, and a band corresponding to the enzyme was cut out for use in a sample for analyzing N-terminal amino acid sequence. Similarly, gel after electrophoresis was stained with CBB and a band corresponding to the enzyme was cut out for use in a sample for analyzing the inside amino acid sequence. The amino acid sequence of these samples were analyzed and the analyzed results are shown in FIG. 13.

2. Extraction of Genome DNA from Streptomyces murinus IFO14802

In accordance with the procedure in the above-mentioned Example, Streptomyces murinus IFO14802 was cultured, and the cultured solution was filtered by using a Buechner funnel and a Nutsche filtering flask so as to obtain a fungus body. The fungus body (1 g) was suspended in 10 ml of TE (10 mM Tris-HCl (pH 8.0) and 1 mM EDTA) solution containing 4 mg/ml lysozyme (Roche Diagnostics), and 2 mg/ml Achromopeptitase (Wako Pure Chemical Industries) and was subjected to bacteriolysis at 30° C. for one hour. To the suspension, 2.4 ml of 0.5 mM EDTA solution was added, and 260 μl of 10 mg/ml Pronase E (Kaken Kagaku) was added and further subjected to bacteriolysis at 30° C. for 5 minutes. Then, 1.4 ml of 10% SDS solution was added, mixed up and down, and incubated at 37° C. for two hours. Phenol (12 ml), which had been equilibrated with a TE solution containing 0.1 M NaCl, was added and stirred for five minutes. Thereafter, 12 ml of chloroform was added and stirred for further five minutes. Then, centrifugation (at 1,500 g at room temperature for five minutes) was carried out so as to obtain the supernatant. This operation was repeated twice. To the obtained supernatant, 72 μl of 10 mg/ml RNase A (SIGMA ALDRICH JAPAN) was added and incubated at 37° C. for one hour. Five M NaCl (4.5 ml) was added and mixed, then 11.25 ml of 30% polyethylene glycol 6000 (Wako Pure Chemical Industries) solution was added and mixed, and allowed to stand at 4° C. overnight. Precipitated genome DNA was spooled out by using a Pasteur pipette, then washed with 70% ethanol and air-dried. Thereafter, the genome DNA was dissolved in 1 ml of TE solution so as to obtain about 1 mg/ml of genome DNA solution.

3. Design of Synthetic Primer

By taking the analysis results of the above-mentioned internal amino acid sequence into consideration, the below-mentioned synthetic primers (Invitrogen) were designed.

Primer IS-F 5′-TTC GGI GAG GTI ACI GCI CGI CAY MG-3′ (SEQ ID NO: 6) Amino acid sequence, N-terminus-F G B V T A R H R G-C-terminus (SEQ ID NO: 25), Primer IS-R 3′-CTY CGI CTR CTG CCI CTG CGI CTC AA-5′ (SEQ ID NO: 7) Amino acid sequence, N-terminus-B A D D G D A B F R-C-terminus (SEQ ID NO: 26).

4. Production of Probe for Southern Hybridization and Colony Hybridization

A PCR reaction was carried out by using the above-mentioned genome DNA derived from Streptomyces murinus IFO14802 as a template. By using TaKaRa LA Taq™ with GC buffer (Takara Shuzo Co., Ltd.), 2 μM of the above-mentioned synthetic primers IS-F and IS-R and 50 ng of genome DNA as a template were added to a reaction system. As the reaction, incubation at 96° C. for 3 minutes was carried out, then a cycle of incubation at 96° C. for 45 seconds, 66° C. for one minute and 72° C. for 3 minutes was repeated 35 cycles, and finally incubation at 72° C. for 10 minutes was carried out. After agarose electrophoresis was carried out, about 240 bp of DNA fragment, which had been specifically amplified by using GENECLEAN™III (BIO101), was extracted. The extracted DNA fragments were subcloned to pGEM™-T Easy (Promega), followed by extracting the insertion DNA fragment again. Then, DIG label was added by using DIG High Prime (Roche Diagnostics) and a probe of an AMP deaminase gene was obtained.

5. Southern Hybridization

Agarose electrophoresis was carried out, in which 6 μg of genome DNA that had been completely digested with arbitrary restriction enzyme was applied for each lane. The DNA was treated with 0.25N HCl solution for 30 minutes, neutralized with buffer used for electrophoresis and then was blotted to Zeta-Probe™ membrane (Bio-Rad) by alkaline blotting using 0.4N NaOH solution. Transcription was carried out by using 2016 VacuGene (Pharmacia LKB Biotechnology) at degree of vacuum of 50 cm·H₂O for 90 minutes. After blotting, the membrane was washed with 2×SSC (NaCl 0.3M, sodium citrate 33.3 mM) solution and air-dried, followed by incubating at 80° C. for 30 minutes, and DNA was immobilized to the membrane. The immobilized membrane was subjected to Southern hybridization by using the above-mentioned probe. The detection was carried out by using DIG Nucleic Acid Detection Kit (Roche Diagnostics). Thus, a restriction map of the enzyme gene was produced (FIG. 14).

6. Colony Hybridization

Genome DNA (12 μg) was completely digested by using a restriction enzyme Not I (Takara Shuzo Co., Ltd) and then subjected to agarose electrophoresis so as to cut out a DNA fragment having a DNA chain length of 3.8 kbp. Then, extraction was carried out by using GENECLEAN™III (BIO 101) so as to obtain an insert for producing a library. At the same time, pBluescriptII KS(+) (Stratagene) was completely digested by using Not I (Takara Shuzo Co., Ltd) and then dephosphorylated by using Alkaline Phosphatase (Takara Shuzo Co., Ltd) so as to obtain a vector for producing a library. The insert and vector were ligated by using Ligation Kit ver. 2 (Takara Shuzo Co., Ltd) and transformed into an Escherichia coli DH5 strain competent cell (TOYOBO CO., LTD) by Hanahan method (J Mol Biol. 1983 Jun. 5; 166(4):557-80, Hanahan D., Studies on transformation of Escherichia coli with plasmids). The obtained clones were planted on a LA plate (ampicillin (SIGMA ALDRICH JAPAN) 100 μg/ml) so that about 500 colonies per one plate were formed and incubated at 37° C. overnight so as to grow the colonies. The grown colonies (total number of about 9500) were lifted to Nylon Membranes for Colony and Plaque Hybridization (Roche Diagnostics) and DNA was immobilized on the membrane. Colony hybridization was carried out by using the aforementioned probe and a colony showing a strong signal was detected by using DIG Nucleic Acid Detection Kit (Roche Diagnostics). The above-mentioned operations were carried out according to the protocol attached to the used reagent. The thus obtained clone was named pSAD and defined as a clone containing an AMP deaminase gene derived from Streptomyces murinus IFO 14802.

7. Analysis of Nucleotide Sequence of AMP Deaminase Gene

Firstly, the analysis of nucleotide sequence was started from the outside of the insert by using M13 Primer M4 and M13 Primer RV (Takara Shuzo Co., Ltd). DNA fragments used for the isolated clone and the probe were subjected to the analysis of nucleotide sequence again by designing a synthetic primer (SIGMA Genosys) based on the sequence of the enzyme gene that had been clarified from the results of the analysis. By primer walking in which these operations were repeated, a full-length DNA sequence was analyzed. The analysis of nucleotide sequence was carried out by using BigDye™ Terminator v3.1 Cycle Sequencing Kit and dGTP BigDye™ Terminator v3.0 Cycle Sequencing Ready Reaction Kit (Applied Biosystems Japan). For analysis, ABI PRISM™ 310 Genetic Analyzer (Applied Biosystems Japan) was used. Synthetic primers used for the sequence analysis were shown below.

MAD-F1: 5′-AAGCAACTCGCCGACCAG-3′ (SEQ ID NO: 8) MAD-F2: 5′-TGGTCCATGCAGGACTTC-3′ (SEQ ID NO: 9) MAD-F3: 5′-CTGGAGAACTACAGCCTC-3′ (SEQ ID NO: 10) MAD-F4: 5′-CAGATCCTCGGCGTCAAG-3′ (SEQ ID NO: 11) MAD-F5: 5′-CTCCAGTACGCCTTCCTG-3′ (SEQ ID NO: 12) MAD-F6: 5′-GTCGGGTCCTGGACACCG-3′ (SEQ ID NO: 13) MAD-F7: 5′-TATACCGTCCGGTAGGTC-3′ (SEQ ID NO: 14) MAD-F8: 5′-GGACAGGAAGACGGACAC-3′ (SEQ ID NO: 15) MAD-F9: 5′-GATTGGCCGAGAAGTACG-3′ (SEQ ID NO: 16) MAD-R1: 5′-TGGTCGGCGAGTTGCTTG-3′ (SEQ ID NO: 17) MAD-R2: 5′-CAGGGACAGGACACTGAG-3′ (SEQ ID NO: 18) MAD-R3: 5′-GATGTCGACATGGCCCTG-3′ (SEQ ID NO: 19) MAD-R4: 5′-CCCAGTCGATCGCGTGAG-3′ (SEQ ID NO: 20) MAD-R5: 5′-TGGTCGTTCCGTGAAGGC-3′ (SEQ ID NO: 21) MAD-R6: 5′-CTCGAACGCCGCGAACGC-3′ (SEQ ID NO: 22) MAD-R7: 5′-CTGGGTGTGCGCGATGTC-3′ (SEQ ID NO: 23) MAD-R8: 5′-CACCATCATCGCCACCTG-3′ (SEQ ID NO: 24)

All nucleotide sequences identified as a result of the analysis were shown in FIGS. 21 to 22 and FIGS. 25 to 26. Furthermore, by annotation using homology search and motif search, a promoter region, a coding region (SEQ ID NO: 4), and a terminator region were clarified (FIGS. 21 to 22). The sequence of the promoter region is shown in FIG. 27, the sequence of the coding region is shown in FIG. 28, and the sequence of the terminator region is shown in FIG. 29, respectively. Furthermore, a deduced amino acid sequence is shown in FIG. 23 (which does not contain signal peptide, SEQ ID NO: 1) and FIG. 24 (which contains signal peptide, SEQ ID NO: 2).

8. Obtaining of Vector pIJ702 for Streptomyces

Streptomyces lividans 3131 (ATCC 35287) carrying plasmid pIJ702 was cultured at 30° C. for two days under the following medium conditions.

YEME Medium+0.5% Glycine+50 μg/ml Thiostrepton (Wako Pure Chemical Industries)

yeast extract 3 g peptone 5 g malt extract 3 g magnesium chloride 1 g glucose 10 g saccharose 340 g glycine 5 g 50 mg/ml thiostrepton solution 1 ml/L (pH 7.0) (dimethylsulfoxide solution)

Cultured medium (200 ml) was centrifuged (at 12,000 g at 4° C. for 10 minutes). The obtained fungus body was suspended in 10 ml of TE-Sucrose (50 mM Tris-HCl (pH 8.0), 10 mM EDTA and 25% Sucrose). Next, 2 ml of TE-Sucrose containing 30 mg/ml of lysozyme (Sigma Aldrich Japan K.K.) and 4 ml of 0.25 mM EDTA solution were added and incubated at 37° C. for 30 minutes. After incubation, 2 ml of 20% SDS solution was added, further 5 ml of 5M NaCl solution was added, gently stirred, and then incubated at 0° C. overnight. Next, to the supernatant obtained by centrifugation (at 100,000 g at 4° C. for 40 minutes), 30% polyethyleneglycol 6000 (Wako Pure Chemical Industries) solution was added so that the final concentration became 10%. Then the solution was incubated 0° C. for 4.5 hours. Thereafter, the solution was centrifuged (at 900 g at 4° C. for 5 minutes) and the obtained precipitations were dissolved in a TE solution containing 50 mM NaCl. Then, 16.8 g of cesium chloride and 1.2 ml of solution prepared by dissolving ethidium bromide in a TE solution so that the concentration became 10 mg/ml were added. The mixed solution was centrifuged (at 1,300 g at room temperature for 15 minutes) so as to remove residues. Thereafter, centrifugation (at 230,000 g at 20° C. for 12 hours) was carried out again. After centrifugation, a plasmid DNA layer was obtained under ultraviolet irradiation. Next, extraction was carried out by using a TE solution saturated with butanol so as to remove ethidium bromide. This extraction was repeated three times. The obtained plasmid DNA solution was subjected to dialysis by using TE as an external solution of dialysis at 4° C. overnight. Thereafter, extraction treatment with a TE solution saturated phenol was carried out once and extraction treatment with chloroform isoamyl alcohol was carried out twice. Next, 1/10 volume of 3M sodium acetate (pH 5.2) solution and two volume of ethanol were added and allowed to stand at −80° C. for 30 minutes. Thereafter, precipitations were collected by centrifugation (at 12,000 g at 4° C. for 15 minutes), washed with 70% ethanol and dried. This was dissolved in 200 μl of TE solution. The final amount of DNA that had been obtained by the above-mentioned operation was about 10 μg.

9. Construction of Shuttle Vector pSV1

Firstly, a DNA fragment obtained by digesting Escherichia coli vector pUC19 (Takara Shuzo Co., Ltd) with restriction enzyme Bam HI and a DNA fragment containing a thiostrepton resistant gene (tsr) obtained by digesting Streptomyces vector pIJ702 with Bcl I (Takara Shuzo Co., Ltd) were prepared. These were ligated to each other by using DNA Ligation Kit Ver.2 (Takara Shuzo Co., Ltd) so as to produce pUCTSR. Next, a DNA fragment (long fragment) obtained by digesting pUCTSR with Kpn I and Cla I (Takara Shuzo Co., Ltd) and a DNA fragment (short fragment) obtained by digesting pIJ702 with Kpn I and Cla I (Takara Shuzo Co., Ltd) were prepared and they were ligated to each other by using DNA Ligation Kit Ver.2 (Takara Shuzo Co., Ltd), followed by transformation to Escherichia coli DH 5 strain (TOYOBO Co., Ltd.). A plasmid, in which pUC19 fragment and a pIJ 702 fragment carried by the thus obtained transformant were ligated to each other, was made to be a shuttle vector pSV1, which was used for the later operation (FIG. 15).

10. Construction of Expression Vector pSVSAD

The gene fragment was prepared by digesting pSAD carrying an AMP deaminase gene with restriction enzyme Not I (Takara Shuzo Co., Ltd). Furthermore, a vector fragment was prepared by digesting shuttle vector pSV1 with restriction enzyme Xba I. Both fragments were blunted at their ends by using DNA Blunting Kit (Takara Shuzo Co., Ltd) and made to be an insert fragment and a vector fragment, respectively. A vector fragment derived from pSV1 was further subjected to dephosphorylation treatment by using Alkaline Phosphatase (Takara Shuzo Co., Ltd). The insert and vector were ligated to each other by using DNA Ligation Kit Ver.2 (Takara Shuzo Co., Ltd), and thereafter transformed to Escherichia coli DH5 strain (TOYOBO Co., Ltd.). The thus obtained plasmid was made to be an expression vector pSVSAD for use in transformation (FIG. 16).

11. Preparation of Streptomyces lividans TK24 Protoplast

Streptomyces lividans TK24 is a strain having resistance against streptomycin derived from Streptomyces lividans 66 and it was provided by D. A. Hopwood (John Innes Institute, Colney Lane, Norwich NR47UH, U. K.). Streptomyces lividans TK24 was cultured using a YEME medium (0.5% glycine) at 30° C. for two days. The 200 ml of cultured medium was centrifuged (at 1,300 g at room temperature for 10 minutes). The obtained fungus body was suspended in 72 ml of 0.35 M saccharose solution. Then, this suspension was centrifuged (at 1,300 g at room temperature for 10 minutes) and the fungus body was suspended again in 60 ml of P buffer solution containing 1 mg/ml of lysozyme (Sigma Aldrich Japan K.K.). The suspension was incubated at 30° C. for 2.5 hours. The suspension after incubation was filtrated with absorbent cotton so as to remove residues. Then, the resultant filtrate was centrifuged (at 1,300 g at room temperature for 10 minutes) and the sediment was washed with 25 ml of P buffer solution. This washing was repeated twice and precipitation was suspended in 1 ml of P buffer solution to obtain a protoplast suspension.

P buffer solution TES [N-Tris(hydroxymethyl)methyl-2-aminoethane 5.73 g sulphonic acid] Saccharose 103 g Magnesium chloride 2.03 g Potassium sulfate 0.5 g Calcium chloride 3.68 g Trace element solution 2 ml/L (pH 7.4)

Note here that 1% monobasic potassium phosphate solution was prepared separately, which was added in the amount of 1 ml per 100 ml P buffer solution immediate before use.

Trace element solution Zinc chloride 40 mg Ferric chloride 200 mg Cupric chloride 10 mg Manganese chloride 10 mg Sodium tetraborate 10 mg Ammonium molybdate 10 mg/L

12. Transformation of Streptomyces lividans TK24

Each of the following solutions was mixed so that the total amount became 121 μl.

TE solution containing AMP deaminase expression plasmid  1 μl pSVSAD (4 μg) Streptomyces lividans TK24 protoplast 100 μl 0.35 M saccharose solution  20 μl

Then, 1.5 ml of P buffer solution containing 20% polyethylene glycol 1000 was added and gently mixed by pipetting. The mixture was allowed to stand for two minutes at room temperature. The mixture was centrifuged (at 1,700 g at room temperature for 10 minutes) to collect precipitate. Protoplast obtained as precipitate was washed twice with P buffer solution. The pellet was resuspended in 0.3 ml of buffer solution P, and then 100 μl each of the suspension was dropped on an R-2 medium. Then, the R-2 top agarose medium that had been kept warm at 55° C. was poured into the plate in the amount of 3 ml/plate so that protoplast was dispersed over the entire plate. The plate was dried in a clean bench for two hours until the top agarose was solidified. After drying, the plate was cultured at 30° C. for 16 hours. Then, 3 ml of R-2 top agarose medium containing 200 μg/ml thiostrepton was added to the plate so as to cover the surface of the plate, followed by drying for two hours. The plate was cultured for further four days at 30° C. so as obtain a transformant (SAD-1) having a thiostrepton resistant property.

The R-2 medium was produced by preparing the following R-2/A and R-2/B separately and combining them. A medium containing agar was R-2 plate and a medium containing agarose was R-2 top agarose medium. When a plate medium was produced, R-2/A and R-2/B were mixed and furthermore, 1% KH₂PO₄ was mixed at the ratio of 1 ml per 200 ml of the final volume.

R-2/A Potassium Sulfate 0.5 g Magnesium Chloride 20.2 g Calcium Chloride 5.9 g Glucose 20.0 g Proline 6.0 g Casamino Acid 0.2 g Trace Element Solution 4.0 ml Agar 44.0 g/L or agarose 5.0 g/L

R-2/B TES 11.5 g Yeast Extracts 10.0 g Saccharose 203 g/L (pH 7.4)

13. Culture of Transformant

In accordance with the procedure of the above-mentioned Example (<Preparation method of enzyme solution>), Solpee N.Y. (2%), Meast P1G (0.5%), KH₂PO₄ (0.1%), MgSO₄ (0.05%) and soluble starch (3%) were added so as to adjust to pH 5.7 and sterilized at 121° C. for 30 minutes. Transformant SAD-1 and a host strain Streptomyces lividans TK24 of transformation was inoculated, pre-cultured for two days and cultured for five days at 30° C. A part of the culture supernatant was collected and used as a sample for measuring the AMP deaminase activity.

14. Measurement of AMP Deaminase Activity

The enzyme activity of the sample obtained above was measured in accordance with the method show in Example (<Method of measuring enzyme activity>). To 1.5 ml of solution obtained by mixing 0.017M 5′AMP-2Na and 1/15M phosphate buffer solution (pH 5.6) at the ratio of 1:2, 0.5 ml of sample solution was added so as to obtain a reaction solution, which was reacted at 37° C. for 15 minutes. After 15 minutes, 2% perchloric acid solution was added so as to stop the reaction and 100 μl of the solution was taken out. Water was added to the solution so that the total amount was 5 ml. Then, OD₂₆₅ was measured. The value similarly measured at reaction time of 0 minute was defined as a blank. Under the below-mentioned conditions, a case where an absorbance difference is reduced by 0.001 during 60 minutes was defined as one unit. The measurement result is shown in FIG. 17. Note here that a culture supernatant obtained after Streptomyces lividans TK24 had been cultured under the same conditions was made to be a control.

As show in FIG. 17, a culture supernatant of transformant SAD-1 exhibited a high AMP deaminase activity. The results confirmed that an AMP deaminase gene was successfully obtained. In addition, it was shown that an AMP deaminase production system was actually constructed by using the gene.

15. Thermostability of AMP Deaminase Derived from Transformant (SAD-1)

The thermostability of AMP deaminase produced by transformant (SAD-1) was measured. Culture supernatant of SAD-1 prepared by the above-mentioned method was treated at a predetermined temperature, and the residual AMP deaminase activity was measured (pH 5.6). Note here that treatment temperature was 30° C., 40° C., 50° C., 60° C., 65° C., 70° C., and 75° C. Furthermore, the treatment time was 30 minutes.

The measurement result is shown a graph of FIG. 18. It is found that the thermostability similar to that of AMP deaminase derived from Streptomyces murinus shown in FIG. 1 was exhibited.

16. Substrate Specificity of AMP Deaminase Derived from Transformant (SAD-1)

In order to determine whether the deaminase derived from transformant (SAD-1) obtained as mentioned above was AMP deaminase or Adenosine-deaminase, the substrate specificity was investigated. The results are shown in FIG. 19. Note here that relative activities when the enzyme activity with respect to 5′ AMP was defined as 100% are represented. The enzyme acted on 5′ AMP most favorably. Furthermore, the enzyme also acted on 3′-AMP, 5′-dAMP, ADP, ATP, Adenosine and 3′5′-cyclic AMP, but did not act on 2′-AMP and adenine at all. In particular, the enzyme acted on 5′-dAMP, ADP and ATP favorably. From the above-mentioned results, it was confirmed that deaminase derived from transformant (SAD-1) was similar to AMP deaminase derived from Streptomyces murinus and it was AMP deaminase. Furthermore, it was determined that the enzyme can be preferably used with respect to the reaction using 5′-dAMP, ADP and ATP as a substrate.

17. Confirmation of Production of AMP Deaminase Derived from Transformant (SAD-1)

The culture supernatant of SAD-1 was subjected to SDS-PAGE (CBB staining) so as to confirm that the enzyme was produced by a transformant. The result of SDS-PAGE is shown in FIG. 20. Lane II shows a control (culture supernatant of host bacteria). Lane III shows a culture supernatant of transformant (SAD-1). In lane III, band that is not observed in lane II is present, showing that new enzyme protein is produced by transformation. In lane I, a band of the protein molecular weight marker is shown. Phosphorylase b (M.W. 97,000), bovine serum albumin (M.W. 66,000), ovalbumin (M.W. 45,000), carbonic anhydrase (M.W. 30,000), trypsin inhibitor (M.W. 20,100) bands are shown from the side of the high molecular weight.

As mentioned above, the present inventors have succeeded in cloning a gene encoding AMP deaminase derived from Streptomyces murinus. Furthermore, they have succeeded in obtaining a transformant in which the gene was introduced by using a transformation system of Streptomyces and confirmed the expression of the gene. These results enabled the enzyme to be produced as a recombinant. Therefore, the stable supply of the enzymes can be realized. Furthermore, improvement of the productivity of the enzyme by gene recombination and improvement of the enzyme itself can be realized. For example, the following (1) to (3) can be provided: (1) improvement of productivity by the use of a promoter exhibiting a high productivity; (2) production of a highly productive transformant by the use of a highly productive strain as a host and construct of a highly productive system using the same; and (3) improvement of productivity, improvement of stability and/or improvement of substrate specificity by modifying a nucleotide sequence and amino acid sequence.

INDUSTRIAL APPLICABILITY

AMP deaminase of the present invention has excellent thermostability. Therefore, the AMP deaminase is suitably used for applications in which a reaction at high temperature is desired. For example, AMP deaminase of the present invention can be used as an enzyme for enhancing the taste in the production of yeast extract or an enzyme for producing a seasoning agent, 5′-inosinic acid.

On the other hand, by using characteristics that AMP deaminase acts on a substrate other than AMP, AMP deaminase of the present invention can be used in various reactions using a substrate such as ADP, ATP, 5′dAMP as a starting material.

The present invention is not limited to the description of the above embodiments and Examples. A variety of modifications, which are within the scopes of the following claims and which are achieved easily by a person skilled in the art, are included in the present invention.

All of the articles, publication of unexamined patent application, and Patent Gazette cited herein are hereby incorporated by reference. 

1. An isolated AMP deaminase having the amino acid sequence of SEQ ID NO: 1 comprising the following characteristics: (1) action catalyzing a reaction of acting on 5′-nucleotide including adenosine as a component so as to deaminate the 5′-nucleotide; (2) substrate specificity acting on 5′-AMP, 5′-dAMP, ADP, ATP, and 3′, 5′-cyclic AMP, which are 5′-nucleotides including adenosine as a component, and acting on 5′-AMP most favorably; (3) optimum temperature having an optimum temperature of around 65° C.; (4) temperature stability being stable at a temperature of 65° C. or less; (5) optimum pH having an optimum pH of around 5.6; (6) pH stability being stable at pH 6.0 to pH 8.5; and (7) molecular weight having a molecular weight of 48,000±2,000 in gel filtration and 60,000±3,000 in SDS-PAGE.
 2. The isolated AMP according to claim 1, wherein said 5′-nucleotide including adenosine as a component is 5′-adenylic acid.
 3. The isolated AMP deaminase according to claim 1, wherein the AMP deaminase is purified from Streptomyces.
 4. The isolated AMP deaminase according to claim 3, wherein the Streptomyces belongs to the genus Streptomyces.
 5. The isolated AMP deaminase according to claim 3, wherein the Streptomyces is Streptomyces murinus.
 6. An isolated AMP deaminase consisting of the following (a) or (b): (a) a protein having the amino acid sequence of SEQ ID NO:1; (b) a protein having an amino acid sequence obtained by deleting, substituting, inserting or adding one to 25 amino acids in the amino acid sequence of SEQ ID NO:1, and functioning as AMP deaminase. 